The present invention relates to a track tensioning system for a vehicle crawler, and to other improvements relating to the crawler itself; vehicles (especially mobile lift cranes) using the crawlers; and methods of making and utilizing the crawlers and other crawler improvements.
Many vehicles, especially construction equipment that moves over uneven ground surfaces, use track drives to maneuver. The track drive is typically part of a crawler, which has a frame on which the endless track is mounted, along with a drive motor. A drive tumbler, also sometimes referred to as a drive sprocket, transmits torque from the drive motor to the track to generate a tractive force, which moves the crawler frame, and thus the vehicle it is a part of, relative to the portion of the track on the ground, and hence moves the vehicle relative to the ground. A track may be powered by a single tumbler, and be trained over an idler wheel also mounted to the frame, or the track may be powered by two drive tumblers. Some crawler frames mount two idler wheels at opposite ends of the frame, and include a drive tumbler elsewhere on the crawler.
In addition to straight forward movements, turning movements are also performed with a set of track drives on opposite sides of the vehicle. These turning movements require one track to turn faster than the other, or more typically, for one track to turn while the track on the other track is stationary. Sometimes the vehicle, particularly construction equipment, will need to turn in place, which can be accomplished by driving one track in one direction and the other track in the opposite direction.
The crawler frame may be a permanent part of the vehicle frame, or in the case of large vehicles that cannot be transported over the highway as a single unit, such as large mobile lift cranes, the crawler is connected to the rest of the vehicle in a manner that the entire crawler, including the crawler frame, track and drive tumbler, is disconnected from the remainder of the vehicle. For some large crawlers, even this disengaged unit will exceed highway transportation weight limits. In that case, the track may need to be removed from the frame to provide individual transport loads within highway weight limits.
The size of the individual components of the crawler is a function of the loads that will be imposed on the crawler, both loads resulting from the weight of the vehicle and any load carried by the vehicle, and loads required to achieve desired vehicle movement, and how those loads are dealt with in the arrangement of the crawler parts. For example, with a track that is made of individual track shoes connected together, which have a driven surface that engages a drive surface on the tumbler, the drive and driven surfaces are part of structures (such as teeth or lugs) that must be large enough to carry the compressive loads required to transmit the torque from the tumbler to the track. Especially with cutting maneuvers (when a crane is changing direction), the amount of tractive force that must be transmitted to the track by the tumbler is very large. If teeth on the tumbler engage lugs on the track shoes, both the teeth and the lugs must be sized to carry the force between the tumbler and the track shoes to produce this tractive effort.
Using conventional tumbler designs, the teeth on the tumbler and the lug on each shoe must carry the entire tractive force. The material costs and weight for using large teeth is multiplied by the number of teeth on the tumbler. More significantly, increased lug sizes are replicated on the lug of every track shoe. There may be as many as 50 or more shoes making up a track. The increased weight is a problem for operation (requiring larger engines and more fuel to operate the equipment) and for transportation between job sites. Therefore a track and tumbler design that could generate the same amount of tractive force with less material weight for the component parts would be a great improvement. Also, for very large tumblers, perhaps weighing 2000 pounds or more, casting the tumbler becomes a challenge, and there are relatively few facilities that can even cast something that large.
As noted above, it is often required on large crawlers to remove the track from the crawler frame in order to get separate loads that are within highway weight limits. This requires that the track be disconnected so that it can be removed from the crawler frame, and then reinstalled on the crawler frame and reconnected at the new job site. Previous methods of reconnecting the ends of the track have proven to be time consuming. With large cranes, and heavy tracks, the reconnection of the track ends may be a daunting task. Also, once the track is replaced on the frame and reconnected, the tension in the track needs to be established for proper track operation. Also, even if the track is never removed, wear in the track elements over time will result in an effective greater length for the track. While individual track shoes may be removed when the track gets too long, it is important to be able to set the correct tension in the track for the varying lengths of the track.
When the track is entrained about one drive tumbler and one idler wheel at opposite ends of the crawler frame, it has been conventional to make the idler wheel position adjustable relative to the rest of the crawler frame to achieve correct track tension. This type of tensioning system may work when the only loads on the adjustability structure are compressive loads, since the idler wheel only transmits compressive loads to the frame. However, if both ends of the crawler frame are equipped with a drive tumbler, then bending loads are carried through the frame as the drive tumbler transmits torque to the track. These bending loads make an adjustable connection of the drive tumbler to the frame, so that the drive tumbler can move horizontally with respect to the rest of the frame to create proper track tension, a complicated structure.